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Fuel Tank Pumps Overheating!

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Fuel Tank Pumps Overheating!

Old 26th Nov 2002, 18:31
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Fuel Tank Pumps Overheating!

Be mindful about keeping an adequate amount of fuel in tanks, especially on 74s. A newfound phenomena about Hydro-Aire (Burbank) fuel pumps capable of reaching temperatures of 1,100 degrees Fahrenheit has triggered an FAA/Boeing Airworthiness Directive: Ordering additional quantities than previously thought of minimum fuel to be maintained in tanks. Only 900 degrees Fahrenheit is necessary to initiate spontaneous combustion of fuel vapors.Emergency Airworthiness Directive
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Old 26th Nov 2002, 18:52
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Whatever happened to the thermal-fuses I seem to remember from a long time ago. They would cut-off the pumps at around 400 degress F.
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Old 28th Nov 2002, 11:37
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GB, excuse a non-pilot butting in, but how useful a control is this when airborne?
 
Old 28th Nov 2002, 13:04
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This takes me back to 120.4's thread a few weeks ago:
What are these airlines who turn up on final at LHR on fumes going to do now?

Carry extra reserves one hopes?
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Old 28th Nov 2002, 14:38
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What's the Likelihood of....

It's beginning to look like an Al Qaeda plot.

I still reckon that, because of the mis-wiring, the pumps are probably finding another earth (courtesy of the short or a further miswiring) - and running on continuously, wet or dry (the cockpit indicator would show it as OFF once power to it via its switch has been cut). Once you short something out it normally overheats; a conductor or terminal strip then melts (usually with a loud bang) and that's what's meant by "shorting out" (assuming CB doesn't do its job). However submerge that same part in fuel most of the time and it doesn't get hot enough for the electrics to melt (fuel rapidly dissipates the heat). What can happen (so the theory goes) is that (instead) the rapid heat build-up caused by the shorting permeates throughout the metal pump and, due to different coefficients of expansion the pump starts binding and so the heat build-up within it increases even more. However as long as it's submerged in fuel, there's enough cold-soak that there's no sufficient localised heat build-up such as to cause any electrical component to melt, "short out" - and so cut the current.

End result is a "blueing" of the metal indicating that extreme localised frictional heat has occurred - mainly due to the binding occurring whilst power remains on to a very hot pump (which is what they've found). And of course in any such shorting situation there would be no CB to overload and blow. Just a theory.

November 27, 2002 - FAA Issues 3rd Warning on Fuel Pumps

WASHINGTON (USA) - The Federal Aviation Administration is requiring about 3,500 Boeing jets to fly with extra fuel to prevent fuel pumps from overheating and possibly causing an explosion.

The FAA has sent airlines three safety warnings about the problem since August. The latest was issued last weekend after pumps in three planes overheated, the FAA said Tuesday.

The warning is intended as a precaution while the FAA figures out what's causing the problem with the pumps, made by Hydro-Aire Inc. of Burbank, Calif.

James McKenna, managing editor of Aviation Maintenance magazine, said the FAA doesn't ordinarily need to make a third attempt to ascertain what's wrong.

"That tends to unsettle the FAA,'' McKenna said.

No serious problems have been linked to the problem. However, if a pump overheats, the right mixture of temperature, oxygen and fuel can cause an explosion, McKenna said.

FAA spokesman Paul Takemoto said the extra fuel eliminates the possibility of an explosion.

"There's no danger of (the pumps) igniting fuel vapors if they're covered,'' he said.

The warning will remain in effect until the agency determines what causes the fuel pumps to overheat, Takemoto said.

The order affects about 1,400 planes - 737s, 747s and 757s flown by U.S. carriers. The FAA is also sending the advisories to its counterpart agencies overseas, covering another 2,100 jets.

The National Transportation Safety Board ruled that an explosion in the fuel tank of TWA Flight 800, a Boeing 747, caused it to crash off the coast of Long Island in 1996, killing 230 people. Vapors in the partly empty tank probably were ignited by a spark in the wiring, the NTSB said.

In August, the FAA ordered airlines to submerge certain Hydro-Aire pumps with fuel or, if not, to X-ray them to make sure they were properly wired. If they weren't, they had to be replaced.

Major U.S. carriers said they had few planes in which the suspect pumps were installed.

In September, the FAA found the problem potentially included every Hydro-Aire pump and ordered all fuel pumps submerged or X-rayed and replaced if faulty.

Then, last week, Boeing Co. received reports of overheating in fuel pumps that had been replaced on a Singapore Airlines 747 and on a Lufthansa 747.

The overheating of those pumps wasn't caused by faulty wiring, said Boeing spokeswoman Cindy Wall.

A pump on a third plane had been removed and was found to have overheated during an inspection.

"It's baffling,'' Wall said. "They've been working nonstop 24 hours a day trying to fix it.''

Hydro-Aire President Greg Ward said the company has made the pumps for 30 years without a problem. It's conducting an extensive investigation that includes chemical analysis of pump parts that overheated, checking airline service records and interviewing retired employees who worked on the pumps, he said.

"We're still piecing together clues,'' Ward said.
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Old 28th Nov 2002, 14:47
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Tuba Mirum

It's something you have to consider BEFORE getting airborne.

sharpshot

They're just gonna have to keep half a tonne of fumes over the pumps!!

As I understand it the fuel can still be used in the event of a diversion/emergency so it's not all exactly wasted extra 'baggage'.

Well, not all the time anyway.
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Old 28th Nov 2002, 15:02
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OVERTALK
Not Bad
Further thought:
I wonder whether Hydro_aire, during their pump redesign process, ignored the phenomenon of conductive silver sulfide build-ups, placed the pos and neg terminal strips too close together and they're consequently being bridged electrically and causing the pumps to run continuously. Whatever the mechanism, it has to be something that's allowing the pumps to both heat up electrically and then "run on" once dry (and also then heating further frictionally due to being overheated). That might make more sense as these pumps are supposedly "fixed" (or replacement) ones - i.e. without the original wire-shorting problems.

The suspicion of Hydro-Aire probably having repositioned positive and negative terminals stems from the root cause of the original shorting problem (too much wiring in close proximity).
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Old 28th Nov 2002, 17:14
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Tuba the immediate precaution is to keep the fuel pumps fully submerged in fuel so as to preclude exposure of the pump housing with vapors (the air inside the tank above the fuel level). A spark is not required for ignition of vapors. Enough heat can cause spontaneous ignition.
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Old 28th Nov 2002, 18:33
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Should be fairly simple to pick up the problem on the 47 Classics - FE just checks for an appropriate electrical load decrease on pump switch off. A bit more difficult on the -400 but can be done with a little organisation.
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Old 28th Nov 2002, 20:09
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I've had a full briefing on the problem, with pretty coloured pictures, and it looks to me just like downright poor design!!

It must be costing a lot of people a packet, and the safety implications are staggering
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Old 28th Nov 2002, 20:14
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Another consequential problem is the lack of understanding by ground crew as to the requirement to leave 500kilos in the centre tank.

The load sheet seems to be incorrect 8/10 times. Our requirement is to turn the pumps off when the centre tank fuel gets down to 500k and thus not use it. We therefore require this 500k to be put into the ZFW (which is all explained before we start). Another problem is the addition of fuel. We supervise the 'dialling in' of fuel before uplift begins as we kept ending up with 500k more than required!

PP
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Old 29th Nov 2002, 03:44
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Questions:
Just out of interest, at what power level is a typical fuel circuit breaker set to trip? i.e. with how many watts do they seem to be reaching 1,100ºF?

Does the whole pump body get "blued"?


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Old 30th Nov 2002, 01:20
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Typical Fuel pumps run at 115VAC, 3 phase, and can draw up to about 3.45KW. (3450 W)
Just because a pump heats up internally does not necessarily trip respective circuit breakers in the cockpit. (A heating element on your stove top gets red hot without tripping a breaker). Usually fuel tank pumps are protected by thermal switches which are supposed to trip, as in the case when a pump is left running in a dry tank and overheats, etc... The question obviously is: Why did the Hydro-Aire pumps reach 1100 degrees without an automatic thermal cutout.

The Thai Airways B73-3 empty center tank explosion at the gate at BKK is believed to have been caused either by its pump overheating, or by arcing of electrical connections.
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Old 30th Nov 2002, 11:24
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In order to allay any fears by those unfamiliar with this problem (on the B747):
1. It is not new.
2. The requirement is to keep the override/jettison pumps (booster pumps by another name) covered with fuel. That keeps them cool and excludes air & fuel vapour.
3. All the fuel in the centre tank may be used. When the contents reach a pre-determined minimum level, the main pumps are switched off and the centre tank emptied into number 2 tank by the scavenge pump which is not affected by the problem.
(so the bad boys can still arrive with sod all fuel but comply with AD 2002-19-52 )
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Old 30th Nov 2002, 12:43
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TG 737 after center tank explosion.
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Old 30th Nov 2002, 15:15
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after reading the ad that faa issued,my impresion was that the blueing was on the bearing shaft,this is not what the thermal fuses are for (they are for an electrical overheat-ie a phase dropped out or the motor seized-fod in the impleler ????-which could be ice or spanners etc) and as such are removed from this equation
the ad also mentioned that the casing had melted in one removed pump indicating again the fault could be on the shaft on the pump motor/bearing,of course this could also be from running the pump excesivly with no load on (ie no fuel in impleller housing)
blueing on metal surfaces (that reach 1100 degs) that are rubbing will be from very localised heat
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Old 2nd Dec 2002, 08:31
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===== Quote ================
GlueBall Date : 29th November 2002 21:20 Typical Fuel pumps run at 115VAC, 3 phase, and can draw up to about 3.45KW (3450 W). …….The question obviously is: Why did the Hydro-Aire pumps reach 1100 degrees without an automatic thermal cutout?
==== End of quote =======

Thanks for the info that the pumps are 3-phase and that the circuit can supply up to 3,750 W.

From this info I assume/infer that when a pump runs dry, it:

(a) does NOT overspeed or otherwise run away, because its synchronous,

(b) ought to draw less current than when pumping, maybe an order of magnitude less power, and

(c) at this low power it could run progressively hotter because of lack of cooling, but fairly slowly, so that all parts of the pump would tend to warm up together.


From AD 022451 I see that:

(a) “… The cause of this overheating is believed to be friction between the pump parts; however, the specific cause of the friction is unknown at this time. …..”

(b) “……. The pump was found with the thermal fuses of the electrical motor winding open …..”, so that in at least one case the thermal fuse worked.

(c) “….. In addition, the aluminum bearing housing adjacent to the back side of the end plate had melted. ….”. Aluminum alloys melt about 1100ºF, so that seems to be good evidence of the temperature.

(d) “……. According to the pump manufacturer, bluing of the steel and/or Stellite materials is evidence of temperatures in excess of 1,100 degrees Fahrenheit. …….. Adjacent areas of the shaft were blackened. ……”. This suggests that most of the shaft did NOT reach this high temperature.


A back-of-the-envelope calculation suggests that if the pump were running dry, and if:

(a) enough friction developed to draw 3,750 W from the power supply, and

(b) the mass of the bearing, adjacent housing, and adjacent part of the shaft was about a pound, and that most of the electrical energy goes into this mass of metal, then that mass could reach 1,100ºF in about one minute (and without tripping the circuit breaker). Perhaps only a few ozs of metal would actually melt.


I wonder sometimes whether there is a little too much trust in the power of liquids to cool.

It is well known to the designers of steam boilers and boiling-water nuclear reactors that once a metal surface in water reaches about 50ºC over the boiling point of the water (I‘m working from memory), a continuous layer of steam is formed which effectively thermally insulates the surface, and the temperature may then jump several hundred degrees before another thermally stable state is reached.

I could imagine this happening to a binding shaft /bearing combination, particularly if only a limited amount of fuel were in contact with restricted flow. A test would have to be done to see if this could happen in practice. If there were no air it might even be safe, though there might be carbon deposits which might cause trouble later.


A nominally dry pump could still have pockets or valleys full of fuel, which could boil off and splash around onto a hot shaft/bearing.


I don’t understand the references to the “end plates of the priming and vapor section” and “scoring was found on the inducer housing of the pump”. Are these two different locations? Is the end plate a (the?) bearing?


Kriskross: A Question: Are the “pretty coloured pictures“ available anywhere for us to look at?


I don’t have any answers. Just questions.


Here is an extract from one of the AD’s:
==== Start of quote from AD 022451 ====
…… override/jettison pump ...... Since scoring was found on the inducer housing of the pump, the pump was disassembled, at which time evidence of severe overheating of the priming and vapor pump section was found. The Stellite (cobalt/chrome/nickel/iron alloy) end plates of the priming and vapor section of the pump were blued and cracked from thermal stress, and the steel pump shaft in that same area was also blued. According to the pump manufacturer, bluing of the steel and/or Stellite materials is evidence of temperatures in excess of 1,100 degrees Fahrenheit. It is believed that such temperatures could only be reached during dry running of the pump. ......... Adjacent areas of the shaft were blackened. In addition, the aluminum bearing housing adjacent to the back side of the end plate had melted.

The other pump was an override/jettison pump ........... The pump was found with the thermal fuses of the electrical motor winding open, which indicates an overheat condition in the motor section. Such overheating may be due to the pump rotor dragging or locking. Evidence of severe overheating of the priming and vapor pump section was found. The end plates of the priming and vapor section of the pump were blued and cracked from thermal stress, and the pump shaft in that same area was blued.
==== End of quote from AD 022451 ====



Last edited by PickyPerkins; 3rd Dec 2002 at 20:56.
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Old 3rd Dec 2002, 07:51
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Now consider the case where a crew has smoke in the cockpit scenario and must urgently dump and land.

As I understand it, there's nothing to stop them from (per the checklist) dumping all that centre-section fuel. BUT the real hazard is that those faulty pumps are then running helter-skelter at the high jettison rate and will be pumping all that centre-section fuel overboard. Anybody see the flaw in that for a good old run-of- the-mill "smoke, so dump fuel and land ASAP situation"????? Very hot dump-pump dry-running in empty CWT? Nor sure if the CWT dump-pumps would shut off before the wing-tank levels reached the preset stop-dump levels (at some later date). Somehow doubt it.

So the jettison action would probably have the jettison pumps "running on" [dry] in an empty CWT. Not good. Would the ensuing fiery accident be put down to a CWT explosion or the smoke in the cockpit emergency - I wonder?

Is this a realistic concern?
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Old 3rd Dec 2002, 12:22
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Overtalk, Belgique,

I think you may have misunderstood the AD and the shorting problem.

It would not be possible for a short in the pump itself to cause the pump to run dry. Because when the pump is switched off, the 3 phase power to the pump is switched off (NOT the earth) via a relay.

For the pump to run dry would require either a failure of the 3 phase relay, or for the crew to forget to switch the pumps off.

As for fuel jettison it is only necessary to dump fuel such that the a/c is below MLW not ALL the fuel.
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Old 3rd Dec 2002, 15:36
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Don't think Belgique was inferring that any fuel dump would be
of "all the fuel". However in the case of a landing on a contaminated runway of marginal length (say), a crew might elect to dump to well below max landing weight. I assume that they could dump down to (for argument's sake) half wing-fuel tanks remaining (for an u/carriage problem for instance).

The question is whether:

a. that would empty the centre-section tank

b. whether the centre-section tank's jettison pumps would run on dry whilst the wing-tank fuel was dumping down to the selected (or preset) level.

c. in which case (a plus b) a hazard might exist from either the electrical shorting or the extreme overheating of the Hydro-Aire pumps.
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